Vertebrate skeletal muscles contain muscle fibers of several types that can be broadly classified as slow or fast on the basis of differences in contraction speed, metabolic activity, and motoneuron innervation. The long term goal of this project is to understand the genetic mechanisms that regulate muscle cell fates - which fiber types muscle precursors form. The first muscle fibers form in the somites of the embryo. Signals from adjacent tissues induce the fates of cells in the somites, including muscle. The proposed studies will characterize this inductive signaling in zebrafish embryos with 3 specific aims: 1. To test the hypothesis that local signals from axial mesoderm specify muscle precursors to form slow rather than fast muscle. (a) Test whether local signaling from axial mesoderm is sufficient to specify the slow muscle fate by transplanting individual fast muscle precursors into the slow muscle precursor position and by transplanting notochord precursor cells into the fast muscle position. (b) Test whether local signaling from axial mesoderm is necessary to specify the slow muscle fate by transplanting individual slow precursors into the fast position. By transplanting cells at various developmental times, these experiments will show when and where muscle precursors are committed to specific fates and the role of local signaling from axial mesoderm. 2. To test the hypothesis that Hedgehog and TGF family members provide competing inductive and inhibitory signals to establish the muscle pioneer cell fate. (a) Test whether endogenous signals inhibit formation of muscle pioneers by transplanting putative inhibitory cells into the slow precursor position. Test whether Radar, a TGF, (b) inhibits the formation of muscle pioneers, (c) blocks the action of Hedgehog, and (d) is required for muscle pioneer patterning. These experiments will establish the role of a putative negative regulator of the muscle pioneer cell fate. 3. To identify genes that regulate specification of muscle cell fates. (a) Examine existing mutations for ones that alter slow muscle, fast muscle, or muscle pioneer cell fates. (b) Identify new genes by screening specifically for mutations that alter the formation or patterning of the three muscle cell types. These experiments will identify the genes that regulate muscle cell-type specification and differentiation.